Extrinsic and Intrinsic Predictors of Variation in Infection by Posthodiplostomum minimum MacCallum, 1921 (Trematoda) in Sunfishes (Lepomis Rafinesque, 1819) from Eastern Ohio

The trematode Posthodiplostomum minimum MacCallum, 1921 (white grub) is a common parasite of centrarchid fishes, frequently reaching high prevalence and intensity in local populations. However, significant variation in infection has been observed across Lepomis Rafinesque, 1819 sunfish species, which are common and abundant hosts for this parasite. Previous observations suggest both extrinsic factors (e.g., habitat-specific characteristics and environmental parameters) and intrinsic factors (e.g., host size, behavior, and susceptibility) as important predictors of infection in this parasite–host system. In the present study, we evaluated the prevalence and intensity of P. minimum in 6 sympatric species of Lepomis sunfish (total of 563 individuals) across 9 lakes in eastern Ohio, U.S.A., that range in surface area from 5.6 to 1,448.7 hectares, and assessed the importance of both extrinsic and intrinsic factors as predictors of infection. We found that collection site (extrinsic factor) and host body mass and species identity (intrinsic factors) were the strongest predictors of infection intensity. Specifically, infection intensity was negatively associated with lake surface area and positively associated host body mass. Lepomis macrochirus (bluegill sunfish) and hybrid sunfish displayed the highest infection intensities, whereas Lepomis cyanellus (green sunfish) and Lepomis gulosus (warmouth sunfish) were significantly less burdened. We were unable to conclude if the observed variation among host species was due to host ecology or susceptibility. These general findings were supported by classification and regression tree (CART) analysis, which optimally partitioned variation in individual host infection intensities by using lake size and host body mass (but not host species identity). Although infection intensity was negatively associated with lake surface area (even among host individuals of the same size and species), the causal mechanisms involved remain unresolved and should be the impetus of future work on this parasite–host system.